This invention relates to gas turbine engines and, more particularly, to a lightweight, efficient mixed-flow exhaust system for utilization on high bypass ratio gas turbofan engines.
Gas turbine engines of the high bypass ratio variety have been proven to demonstrate greater efficiency and higher performance than their turbojet counterparts, particularly in subsonic transport applications. In engines of this type, a core engine gas generator is surrounded by a generally concentric annular bypass duct which carries relatively cool air pressurized by an upstream fan. The bypass ratio of such a turbofan engine is determined by the measure of air flowing through the bypass duct compared to that which flows through the core engine, and the most recently developed commercially available high bypass ratio turbofan engines have bypass ratios in the order of 4 - 7:1. Generally, the bypass flow and core flow are discharged through separate concentric coannular nozzles to generate two additive components of propulsive thrust. In some advanced engines, it has been proposed to mix the two flows together and to discharge the mixture through a common nozzle, since it has been recognized that a propulsive thrust benefit can be obtained in this manner. And, in order to insure that the concentric flow streams are effectively mixed, they must be passed through a flow mixer such as of the well-known multilobed variety prior to being discharged through an exit nozzle.
The problem associated with such mixers is that their propulsive thrust benefits are often outweighed by increases in system hardware cost, complexity and weight (an important design consideration in aircraft applications). Previous efforts on mixer designs for high bypass ratio gas turbofan engines have concentrated on schemes which mixed substantially the total flow of the bypass and core engine streams. For typical turbofan engines of bypass ratios 4 or higher, the mixers became so heavy and the pressure losses such as those due to scrubbing drag and mixing became so large that they were dropped as viable contenders for thrust and performance augmentation.
However, the quest for greater efficiency and improved performance goes on as aircraft become heavier and their operators plan for longer range routes. Consider for a moment that a one percent increase in thrust is equivalent to 500 pounds of thrust in an engine of the 50,000--pound class. Thus, if a higher thrust can be obtained for a given throttle setting or, conversely, the same thrust can be obtained at a lower throttle setting, significant savings in fuel costs can result. Therefore, it becomes well worth the search to find even small improvements and gains in overall engine thrust. However, common sense dictates that these improvements be practical from both the mechanical and economic points of view. For example, there are thousands of gas turbine engines in operation today, many of them being of the high bypass ratio variety. Preferably, any component improvement program would entail only minor, low-cost modifications to these existing engines.